Because copy_only log backups don't truncate the transaction log and hence don't mark the space in the log as reusable. You take all your log backups with copy_only and things will work great for the short time until the log file grows enough to fill the drive.

The point is that a copy_only log backup is not part of the log backup sequence, hence you can delete it without problem. It's not that SQL allows you to skip to the next in sequence, it's that the one you deleted was never part of the sequence at all.